Polyhedral trawl door



March 14, 1967 F. J. LUKETA POLYHEDRAL TRAWL noon Original Filed Aug.24, 1962 10 Sheets-Sheet 1 JNVENTOR. FPA/VK J Jul E A March 14, 1967 F..1. LUKETA POLYHEDRAL TRAWL DOOR l0 Sheets-Sheet 2 Original Filed Aug.24, 1962 INVENTOR. FRANK J lUAEr/l v W. I M

A TTOP/VE'VS'I F. J. LUKETA POLYHEDRAL TRAWL DOOR March 14, 1967 10Sheets-Sheet 5 Original Filed Aug. 24, 1962 INV-EN TOR. FRANK J. zuA e'mMarch 14, 1967 F. .1. LUKETA POLYHEDRAL TRAWL DOOR l0 Sheets-Sheet 4Original Filed Aug. 24, 1962 A r RF. 5/ WV A r ram/5 V6 March 1967 F. J.LUKETA POLYHEDRAL TRAWL DOOR l0 Sheets-Sheet 5 Original Filed Aug. 24,1962 INVENTQR. FRANK J ZUK7A 10 Sheets-Sheet 6 if I F. J. LUKETAINVENTOR. FRANK I]: ZUk/A POLYHEDRAL TRAWL DOOR Original Filed Aug. 24,1962 March 14, 1967 A r roam/5V6 March 14, 1967 F. J. LUKETA POLYHEDRALTRAWL DOOR Original Filed Aug. 24, 1962 ill L 10 Sheets-Sheet 7 INVENTOR. FRANK cf. (U 7 644 A r role/v5 VJ March 14, 1967 F. J. LUKETAPOLYHEDRAL TRAWL DOOR Original Filed Aug. 24, 1962 10 Sheets-Sheet 8INVENTOR. FRANK d? 40/(57/1 &

Rvw 0 2 a, V m

March 14, 1967 F. J. LUKETA 3,308,563

POLYHEDRAL TRAWL DOOR Original Filed Aug. 24, 1962 10 Sheets-Sheet 9INVENTOR. PA /WK r/T 4 0 66 March 14, 1967 F. J. LUKETA POLYHEDRAL TRAWLDOOR l0 Sheets-Sheet 10 Original Filed Aug. 24, 1962 INVENTOR. FPA/VKz/. .(UKEZ'A United States Patent 3,308,568 POLYHEDRAL TRAWL DOOR FrankJ. Lulreta, 5567 Greenwood Ave. N., Seattle, Wash. 98103 Continuation ofapplication Ser. No. 477,283, Aug. 4, 1965, which is a continuation ofapplication Ser. No. 219,276, Aug. 24, 1962. This application June 29,1966, Ser. No. 564,492

Claims. (Cl. 439) This application is a continuation of my now abandonedapplication Serial No. 477,283, filed August 4, 1965, as a continuationof my now abandoned application Serial No. 219,276, filed August 24,1962, as a continuation-in-part of my now abandoned application SerialNo.837,025, filed August 31, 1959.

The present invention relates to trawl doors of the type connected tothe wings or curtains of a trawl net to spread the net when towedthrough the water at trawling speed. Trawl doors according to thepresent invention are designed to operate at an optimum angle of attackto widely spread the net and net wings with minimal drag, and to thisend are equipped with auxiliary hydrofoil surfaces, so related to ahydrofoil shaped posterior surface that the water flow, which wouldotherwise tend to break away from the posterior surface of the door athigh attack angles, is maintained in smooth flow relation to the door,reducing drag and increasing lift. Such lessening of the drag byincreasing the lift enables an adequade lateral spreading of the netswings with a minimum expenditure of towing power. Even a low poweredtrawler can thereby drag a net of large size at appreciable spread, andcatch a maximum number of fish.

Trawl doors according to this invention can be adapted to midwater or tobottom trawling, but the forms of doors shown are designed primarily foruse in bottom trawling, such as in conjunction with a net of the typeshown in my now abandoned copending application Serial No. 362,228,filed April 24, 1964. With respect to such bottom trawl doors, a furtherfeature of the invention has to do with tilting of the doors to improvethe bottoming characteristics thereof.

The elimination of unnecessary drag is a primary object in the devisingof trawl nets and accessories therefor. The more drag that can beeliminated in any trawling component the more eificient the trawlingoperation is. In particular, if the doors used ahead of the oppositewings or curtains of the net can be of such type as to inherentlyproduce only minimal drag, and yet can produce a large lateral componentor lift, the wings or curtains can be spread more widely and can beextended to great lengths, thereby greatly increasing the bottom areathat can be swept and the volume of the bottom space which the netsweeps. The provisions of a trawl door which in a high degree eliminatesunnecessary drag and provides great lateral spreading or sheeringability is a primary object of this invention.

The usual trawl door is planar, and is dragged at a rather large angleof attack relative to the water. This door configuration and attitudeare bound to produce large drag forces. Doors of hydrofoil shape havebeen proposed, but at angles of attack large enough to produce a widespreading, the water tends to break away from the posterior convexsurfaces of the doors, and the eddy currents thereby produced increasethe drag to the point where much of the advantage of the hydrofoil shapeis lost. It is an important object of this invention to provideauxiliary cambered hydrofoil surfaces or surface elements which overcomethis tendency to break away, and which instead deflect the eddy currentsback toward the convex hydrofoil surface, and particularly the afteredge thereof, so as to create the smoothest possible flow of the waterpast the door. Additionally, these auxiliary hydrofoils by increasingthe lateral velocity of the water flow, provide considerable lateralspreading effect or lift, which supplements that of the main hydrofoil.

The maintenance of stability in such doors as they drag along the bottomis a further problem with which the present invention deals, and in apreferred form the door is formed with further surfaces inclined in thevertical sense, which by water reaction tend to improve its uprightstability, its ability to hold to the bottom and to ride with its loweredge in contact throughout with the bottom and also to give itanti-mudding characteristics. Other provisions, mentioned hereinafter,also improve its upright stability or attitude.

The location of anchorage points on such a door, one at least for thetowing warp and one or more, usually two at least, for the sweep andcurtain lines which extend to the net, is an important consideration,and it is an object of this invention so to locate such anchorage meansthat they will create a moment tending to maintain the hydrofoilsurfaces of the door at the correct angle of attack with relation to thedirection of drag. Also, it is an object to locate the single point ofconnection of the towing warp where it will neutralize any tendency ofthe dragging lower edge of the door to tilt the door, and so to disturbits upright stability, but instead will maintain the door either uprightin use, or inclined somewhat backwardly. I

While such doors have been weighted in the vicinity of their lower edge,this normally has been done generally uniformly throughout the length ofthe doors lower edge, but I have found that such distribution of Weightis less advantageous than a concentration of 'weight in a particularlocation, and it is an object of this invention to provide for suchconcentration of the weight as will best stabilize the door fore andaft. Also the weight is made removable whereby it can be adjusted inmass or location if desired.

Many trawl doors are of little thickness, and can be fabricated ofplates of metal or cast of aluminum, while others are of somewhatgreater thickness, and usually are made of wood reinforced by andballasted with metal. The door of this invention is of hollowconstruction, and is made principally of aluminum and rubber.

All such doors of necessity must be bulky and heavy, especially when notsubmerged, and my US. Patent No. 3,006,097, granted October 31, 1961,discloses and claims automatic stanchions for securing trawl doors asthey are hauled in with the net and on board the trawling vessel. Oneobject of the present invention is to provide a trawl door particularlyadapted for cooperation with such automatic stanchions.

Still further, trawl doors as now used are very heavy, particularly whenout of the water, and are difiicult to manage. Oftentimes, conventionaltrawl doors may in corporate considerable wood in their construction, inorder to insure that they will remain correctly upright during settingof the net. This adds to their weight out of water, and detracts fromthe forces with which the doors are held down while dragging the bottom.Another object of this invention is to afford temporary buoyance to theupper part of a trawl door, as it sinks below the surface in setting,and so causes it to assume a correct attitude during setting, yetdispels this buoyance once the door assumes its correct attitude. Thiswill avoid superfluous buoyancy in the water, and superfluous weightduring subsequent hauling operations. All the necessary ballast issupplied by weights of high specific gravity, removably mounted insidethe hollow core 0 the door.

It is desirable so to form a trawl door that it can be inverted top forbottom, and so can be used either as a port-side door or as astarboardside door, and so that when a protective runner shoe on thedoors lower edge is worn by bottom contact the door can be inverted topresent the unworn originally upper edging to the bottom. It is anobject of this invention to permit such inversion, particularly in ahollow door, without disturbance of its balance.

Further objects of this invention deal with the mechanical constructionand strengthening of the door in relation to the stresses to which itwill be subjected, its protection against damage from contact withobstructions on the bottom and the like, all of which, with furtherobjects not specifically mentioned, will be more fully understood -asthis specification progresses.

In the accompanying drawings the invention is shown in alternativeforms, and the principles of this invention will be explaied more fullyhereinafter with reference to those forms, and those features which arebelieved to be novel will be set forth in the accompanying claims.

FIGURE 1 is a side elevation, looking toward the concave face oranterior surface of one form of the door, and FIGURE 2 is a similar viewof the convex posterior surface, with parts broken away.

FIGURE 3 is a view similar to FIGURE 2, but with the cover and otherportions of the door considerably broken back to illustrate the interiorconstruction.

FIGURE 4 is a general longitudinal section through the same door, takensubstantially at the plane indicated by the line 4--4 of FIGURE 3.

FIGURE 5 is a transverse section through the door on a vertical plane,taken substantially at the plane indicated by the line 5-5 in FIGURE 4.

FIGURE 6 is a detail sectional view, enlarged, showing the mounting foran anchorage to the door.

FIGURE 7 is a general sectional view of a modified form of auxiliaryhydrofoil surface elements, and FIGURE 8 is a sectional view, similar toFIGURE 5, of the same elements.

FIGURE 9 is a diagrammatic view similar to a top plan view, illustratingthe effect of the auxiliary hydrofoil surfaces.

FIGURE 10 is a diagram illustrating water flow over a hydrofoil-shapedplate or door at a low angle of attack, and FIGURE 11 is a like diagram,but at a higher angle of attack.

FIGURE 12 is a diagram similar to FIGURE 11, illustrating the eflfect ofthickening the door, and increasing the curvature of the posteriorsurface relative to that of its anterior surface.

FIGURE 13 is a diagram similar to that of FIGURE 12, illustrating theelfect of the added auxiliary hydrofoils in smoothing the flow of waterover the posterior surface, and in increasing the velocity thereof.

FIGURES14 to 31 illustrate a modified form of door.

FIGURE 14 is an anterior face view, similar to FIG- URE 1.

FIGURE 15 is a longitudinal section through the door, similar to FIGURE4.

FIGURE 16 is a vertical section through the door corresponding to FIGURE5, taken at line 16-16 of FIG- URE 15.

FIGURE 17 is an enlarged vertical section of the upper edge of the door,showing details of construction, and FIGURE 18 is a further enlargedvertical section illustrating the means for securement of the protectiveedging.

FIGURE 19 is a detail in section of the mounting of an auxiliaryhydrofoil in a supporting bracket.

FIGURE 20 is a detail in section, illustrating the manner in which theanchorage for a towing warp can be relocated when the door is inverted.

FIGURE 21 is a posterior elevational view, partly broken away, of an endof the door, and FIGURE 22 is a similar but enlarged view, these twoviews dealing with the means to provide temporary buoyance to the doorsupper edge, and FIGURE 23 is an enlarged vertical section at the line23-23 of FIGURE 21.

FIGURE 24 is an isometric view of the anterior surface of a door, fullyassembled, and with its removable edging removed but in position forassembly upon the door.

FIGURE 25 is a detail, in cross-section, showing the manner ofsecurement of the edging, and the back cover, and FIGURE 26 is asectional view at the line 26-26 of FIGURE 25.

FIGURE 27 is an isometric view of the anterior face of the door, fullyassembled and rigged, and FIGURE 28 is a somewhat similar view, with theparts exploded.

FIGURE 29 is an isometric view of the posterior face of the door, fullyassembled and rigged, and FIGURE 30 is a somewhat similar view, with theparts exploded.

FIGURE 31 is a sectional detail showing a ballast weight and the meansfor securing the same in place.

Such a trawl door is towed by a towing warp from the trawling vessel,and is connected to a net trailing behind the door by a sweep line and acurtain line, which transmit the drag to the net. Two such doors areused, so shaped and oriented as to cause the curtains of the net(corresponding somewhat to the wings of a standard trawl net, but muchlonger, and open of mesh) to diverge widely apart and forwardly, so asto sweep the maximum bottom area. The doors themselves are generallyupright in use, and during bottom trawling their lower edges ride evenlyupon the bottom, and so the door must be nonbuoyant, and usually isweighted, not only to hold it down but to maintain it in uprightdisposition as it is set and while it sinks to the bottom, and later toassure that it rides evenly on the bottom. The orientation ofconventional doors, as viewed in plan, is usually maintained at an angleof attack, relative to the direction of drag, by employing a towingbridle of four short heavy chains or links which constitutes anadditional source of undesirable drag, and which spaces the common pointof their anchorage undesirably and considerably outwardly from the door.

The present door comprises a body generally designated by the numeral 1,which in the form shown, as viewed from one side or the other, isgenerally rectangular, though with rounded corners. It may sometimes berounded at its forward end. The door shown, as viewed in plan (seeFIGURE 4) is curved or cambered from its forward end 10 progressivelytoward its after end 19, although the cambered form is not strictlyessential to other novel features. In other words, the body 1 has aconcave anterior surface 11 and a complementally convex posteriorsurface 12, and the door has anchorage means for the lines mentionedabove so located as to produce a moment tending to incline the generalplane of the door (see, for instance, the dash-line P in FIGURE 9) at adesired fairly large angle of attack or with relation to the directionof drag D. For instance, a shackle 2 for attachment of a towing warp Wis itself pivotally mounted upon a fore and aft rib 13 outstanding frombut close to the anterior surface 11 of the door aft of its leading end10, but only slightly ahead of the doors midpoint. Other shackles 3 aremounted at the posterior surface of the door, in a manner that will beexplained more fully hereinafter, these being for connection to sweepand curtain lines indicated, for instance, at S in FIGURE 4. Preferablythe shackle 2 for the towing warp W is mounted slightly below thelongitudinal center line of the door; see FIGURES 1 and 3. Since thebottom of the door drags along the bottom, and so tends to tilt theupper edge of the door forwardly and downwardly, such a location of theshackles anchorage to the towing warp, which trends upwardly, tends 'topull the lower edge of the door forwardly and upwardly, with the netresult that the opposing forces maintain the door generally upright asit is dragged through the water. The upper shackle 3 is located wellabove the lower edge of the door and the lower shackle 3 is locatedclose to the lower edge, both well aft of the location of the shackle 2.The relative location of these forward and after anchorage means is suchthat a moment is created between them, if the door is disoriented, thismoment tending to incline the general plane P of the door at theselected angle a with relation to the direction of drag (see FIGURE 9).The optimum angle of attack or with a door incorporating the principlesof this invention has been found to be approximately 45.

The door is thus oriented in actual use to an appreciable degree inaccordance with the moment produced between the anchorages 2 and 3,between anchorage 2 and the bottom, and between anchorage 2 and themidpoint of the door, and when so oriented its cambered surfaces 11 and12 near the leading end 10 make but a small angle of attack withrelation to the direction-of drag, but with the progressive curvature ofthese hydrofoil surfaces 11 and 12 the angle of attack becomesincreasingly steep, until it reaches quite a high angle adjacent thetrailing edge 19.

Towing is a relatively low-speed operation in trawling, perhaps threeknots for example. With the increasing angle of attack of anylongitudinally cambered surface, towards its trailing end, there is anincreasing tendency for eddy currents to break away from the posteriorsurface thereof. If the surface is at a sufliciently low angle of attack(0c, in FIGURE 10) and the speed is low, the drag as represented by thearrow D is low, but the lift L is only fair, and the door causesinsuflicient spreading of the nets curtains or wings. But if the angleof attack a." is increased, as in FIGURE 11, the lift L" may beincreased, but so also is the drag D", for turbulence is created overmuch or all of the posterior surface, as the arrows T there indicate inFIGURE 11. If a simple cambered plate be replaced by a cambered bodyhaving thickness, and having a curvature at its posterior surface 12sharper or exceeding that at its anterior surface 11, as in FIGURE 12,the lift L" remains unchanged at the same angle of attack 0/, but adecreased drag D increases the efiiciency somewhat. There is still toomuch drag-creating turbulence at T. The posterior surface of the door,which is thickened intermediate its leading and trailing edges, may atits leading edge be presented at almost zero angle to the direction ofdrag, and the relative current, tangent to the leading edge curvature,tends to follow the curvature of the posterior surface as its angle tothe direction of drag increases, but eventually the angle becomes sogreat that the current tends to break away from the surface, andturbulence results. The region of greatest thickness, where turbulenceis most likely to begin, is centrally between leading and trailingedges, well back of the former.

The full advantage of a door cambered or of hydrofoil shape at itsposterior surface, at least, is not realized until there is no tendencyto turbulence at the posterior surface, and this is achieved, accordingto the present invention, by disposing one or more auxiliary camberedhydrofoils 41, 42, 43, 44, etc. (the exact number is not material) atsuccessive spacings behind but relatively close to the posterior convexsurface 12, and at a location where the eddying begins; see FIGURES 9and 13. When this is done, each such auxiliary hydrofoil intercepts eddycurrents, and tends to direct them inwardly, as indicated at E, to causethe water to flow smoothly past the posterior surface 12 as indicated atS, and at somewhat increased lateral velocity. The drag created byeddying is thereby prevented, which allows the door to produce as largea lift L as before, but its decreased drag D lessens the power required,and the increased lateral velocity tends to spread the doors more widelyapart, and so increases efficiency notwithstanding that the generalplane P of the door is at a relatively high angle of attack.

The auxiliary hydrofoils 41, 42, etc. are supported from oppositebrackets 4, which are secured to the body 1 at its posterior surface 12in proper disposition and orientation, as by flanges 4 and bolts 4". Onesuch bracket is located near the upper edge of the door, and the othernear its lower edge, and each can be protected against wear, should oneengage the bottom, by a protective pad 40 of abrasion-resistant rubber.The support of the individual hydrofoils may be accomplished in anyconvenient manner, but as shown each bracket 4 is formed with sockets 45each facing the opposite bracket, within which is received the end of ahydrofoil 41, 42, etc. Each hydrofoil may be secured in its socket 45 bymeans such as are best shown in FIGURE 19, namely, by a carriage bolt 46within a square hole 47 in one wall of its socket 45, and a nut 48received within a large hole 49 in the opposite wall of the socket,clamping the hydrofoil against the first wall. The brackets 4 serve togive directional stability to the door. They may lie in a plane parallelto the bottom, as seen in FIGURES 2, 3, 5 and 8, or they may be slopedaway from the bottom, as in FIGURE 16, to more surely avoid contact withthe bottom.

The door, whether cambered or not, is preferably formed with rearwardlyinclined lower and upper edge portions or bands 14 and 16, respectively,of equal area. These might adjoin one another, at opposite sides of alongitudinal apex line midway between the upper and lower edges, or, asshown, they are separated by an intervening central portion or band 15,which while the door is in use is upright. All such bands are shown asof hydrofoil shape longitudinally. The dihedrally related bands 14 and16, in cooperation with the below-center location of the anchorage ofthe towing warp W to the door, already mentioned, afford stability tothe door in a manner explained below.

The lower edge of the door drags over the bottom, and should bearthereon evenly throughout its length. The towing warp W is anchored tothe doors anterior or concave face at the shackle 2. This shackle 2, asalready explained, is located close into and slightly ahead of the doorslongitudinal midpoint, and somewhat below the longitudinal center lineof the door. Such a belowcenter location produces a tendency for theupward component of the pull of the towing warp to tilt the doorbackwardly about its lower edge, and so counters the drag of the bottomagainst such lower edge. As a result the door tends to remain upright asit advances, and desirably can be so arranged as to tilt rearwardlyslightly. The small longitudinal moment arm from the anchorage 2rearwardly to the midpoint of the doors length tends to preventuptilting of the doors forward end 10 off the bottom, especially whenassisted by the location of ballast weights forwardly, as will beexplained later. The upper dihedral band 16, when the door is tiltedsomewhat backwardly, causes the door, by water reaction, to be stronglyurged downwardly, and therefore it will stay on the bottom more surelywith a minimum of ballast.

Thus there are forces arising from the dihedral disposition of bands 16,14, from the below-center location of anchorage 2, from the lattersshort spacing ahead of the longitudinal midpoint of the door, and fromthe longitudinal disposition of the ballast weights, which cooperate tomaintain the door at the proper angle of attack, yet held down to andbearing evenly along the lower edge upon the bottom, and approximatelyupright as it advances.

Inasmuch as the lower edge bears upon and drags over the 'bottom, wearoccurs there, and should be absorbed by a protective element. Forexample, a pad 7 of wear-resistant rubber is vulcanized or otherwiseheld in place upon the doors lower edge (FIGURES 5 and 8), and can berenewed when worn, or as shown in FIG URES 16, 17, 18, 23, 24 and 25,and as is preferable, a complete ring 7a encircles all the edges of thedoor, and is removably secured in place, so that it is replaceable whenworn at both horizontal edges. The ring 7a may be formed with numerousinwardly directed studs 70 fitting sockets or apertures 70a about therim of the door, and with a flange 71 shouldered at 72, the flange beingpinched between the body structure In and the removable cover 17 by thehold-down bolts 18. A nut 18a (FIG- URES 25 and 26) receives thethreaded end of each bolt 18. The shoulder 72 reacting against the bodystructure In assists in retaining the ring 7a. Screws 77 received inthe'studs 70 expand the latter (FIGURE 18) and so retains them in theirconical holes 7051.

When the protective edging 7a is employed, the trawler can carry aspare, and whenever one becomes unduly worn, it can be replaced withlittle loss of time, whereas with shoes or the like that are bonded inplace, the door as a whole must be shipped to a repair station, andreplacement is bound to be expensive, and a cause of delay.

The door is ballasted at its lower edge. While the location of ballastweight longitudinally would be governed by the doors balance, and by anytendency to tilt upwardly its forward end-the aim being to have it bearequally upon the bottom along its lower edge-the towing warp anchored at2 produces some upward component, especially when wave action or thelike tends to jerk and pull the warp taut. Accordingly, it is preferredthat the ballast be concentrated towards the forward end of the doorslower edge. Ballast weight or weight blocks 6 are shown, so located.

It is preferred that the door be formed as a hollow nonbouyant body In,for instance of cast aluminum, covered by a light-weight back plate orcover 17 held in place by the bolts 18. It must, however, be drainable,and is so shown. In the form of FIGURES 1 to 8 it is provided withtransverse or upright ribs 5 and a longitudinal rib or ribs 50, togetherwith angularly disposed bracing ribs 51, which interconnect theanchorage point for the shackle 2 with the anchorage points for theshackles 3. The two latter points are interconnected again by atransverse rib or spreader 5, so that stresses are transmitted betweenthe points of maximum stress in a most direct manner, and are stronglyresisted. The rubber-coated ballast weight 6 shown in FIGURE 3 ismounted within a cell or cells within the hollow structure of the door,by bolts 60 or the like, and may be removed and shifted in location, asfound necessary, upon removal of cover 17.

The door of FIGURES 1 to 8 is not intended to be reversible top forbottom, whereas the door of FIG- URES 14 to 23 is thus reversible. Theballast weights (in in the latter form are received alternatively incells within the hollow door structure along the upper and lower edges(whichever edge is then the lower one), and are secured therein by thebolts 60a, see FIGURE 31.

Buoyance is afforded, at least temporarily, to the upper portion of thedoor body in any of a number of ways. In the form of FIGURES 1 to 8 thecells defined by interior dividers 5 and 50 are open, at leastrestrictedly, to the atmosphere when the door is out of water, by way ofholes 17b along the upper margin of the body la or its cover 17. Alongthe lower margin of the same, drainage holes 17a communicate with theinterior. When setting the net the initial few moments in the launchingof the door are critical, because unless it automatically assumes itsupright disposition and positive angle of attack it may veer inboard andfoul gear at the opposite side. This will require that it be hauled in,cleared, and reset, with consequent labor and loss of time. The locationof the ballast weight at the lower edge, and the buoyance afforded theupper edge by air temporarily trapped there, because it cannot at onceescape through holes 1717, which holes must be regulated as to size, toprovide the intended lag in escape of air, which, being replaced bywater, causes the door to assume its correct orientation as it sinks inthe water. Air escapes somewhat slowly through holes 171;, for reasonsalready given, and the temporary buoyance is dispelled. In consequencethere is no residual buoyance which must be overcome all during use byproviding extra ballast weights, as must the permanently buoyant meansusually provided along a doors upper edge.

The door in the form of FIGURES 14 to 31, which is reversible top forbottom, accomplishes a like result in a somewhat different manner.Buoyant blocks 8 fit within cellules in the upper portion of the doorsinterior, and are held in place by the cover 17. Apertures in thehorizontal dividers 50 afford communication between drain holes 17a andair exit holes 1711, in cooperation with channels 81 in blocks 8. Suchcommunication is restricted, since the blocks partially mask the holes17 b. When the door is let into the water the ballast weights and thebuoyant upper space causes it to assume the correct attitude, and as thewater slowly displaces the air within the hollow door, it graduallyloses its buoyance, which now is not needed, and if present would bedetrimental, in that it would require added ballast; see FIGURE 23.

It is desirable to provide pivots for the shackle pins at 2 and 3 whichwill avoid wear on elements that are integral parts of the door itself,for if wear on the door parts were permitted, these would have to bereplaced rather frequently, with considerable difficulty and expense, orthe entire door would have to be discarded and replaced. Accordingly, Ihave adopted the construction shown in FIGURE 6. The shackle 2 (and thefollowing applies to shackles 3) has the aligned eyes 20, within whichare disposed bushings 21 which receive the tie bolt 22. The bushingshear at their ends on removable caps 23. The hole 13a in the rib 13 isoversize so that the tie bolt 22 does not bear therein. Rather, the eyes20 pivot around the bushings 21. The bushings 21 and caps 23 arenonrotative, and are held against rotation relative to rib 13 by bolt22. Any wear occurs in the eyes 29, bushings 21 and caps 23, which canbe removed and economically replaced when sufficiently worn.

The reversible door of FIGURES 14 to 31 employs a modified form ofanchorage for the towing warp. Whichever horizontal edge is uppermost,the shackle 2a should be below the longitudinal center line. It must beshifted when the door is inverted. As seen in FIGURE 20 the shackle 2ais formed with a pivot sleeve 29a that has a bushing 21a held by a pin22a between ears 13b of a bracket secured to the anterior face of thedoor body. The shackle 2a is located nearer one end of the pivot sleeveZita than the other end. In the full line position of FEGURE 20 theschackle 2a is well below the center line C of the door, whereas if theshackle and pivot sleeve are inverted to the dot-dash line position, andthe door too is inverted, the shackle is again in the correctbelow-center location.

What is claimed is:

l. A trawl door comprising a nonbuoyant body which is generally uprightin use, and at least the posterior surface of which is of hydrofoilshape in the longitudinal direction, and auxiliary surfaces locatedbehind the posterior face of the door in position to intercept incipienteddy currents and to direct them towards such posterior face and itstrailing end.

2. A trawl door as in claim 1, wherein the auxiliary surfaces are ofgenerally hydrofoil shape, and succeeding auxiliary surfaces are spacedoutwardly from the posterior face of the door, and successively aft ofthe preceding auxiliary surface, when the door is in position of use.

3. A trawl door comprising a nonbuoyant hydrofoil body for dispositionuprightly in use with its general plane at a selected angle of attack,anchorage means at its anterior surface for a towing warp, said anteriorsurface being formed as a hydrofoil which at its forward end makes aselected small angle to the direction of drag, but of progressivelyincreasing angle towards its after end, and the bodys posterior surfacebeing convexly curved longitudinally, generally complementally to but ofsharper curvature than its concave anterior surface, whereby its forwardpart is presented at a lesser angle to the direction of drag than theforward part of its anterior surface, and auxiliary hydrofoil surfacemeans spaced aft of the bodys posterior surface, said auxiliaryhydrofoil surface means being located to intercept eddy currents thatmay tend to break away from the bodys posterior surface, and to directthe same again towards that sur-v face, and towards the after edgethereof.

4. A trawl door comprising a nonbuoyant body for disposition uprightlyin use with its general plane at a selected angle of attack relative toits direction of advance, anchorage means at its interior surface for atowing warp, said anterior surface being formed as a hydrofoil having agiven concavity fore and aft, which at its forward end makes a selectedsmall angle to the direction of advance, but of progressively increasingangle towards its after end, and the bodys posterior surface having aconvexity fore and aft of lesser radius than the concavity of itsanterior surface, but generally similarly shaped, to leave the bodythicker in its portion intermediate its forward and after ends than ateither such end.

5. A trawl door comprising a nonbuoyant body for disposition uprightlyin use with its general plane at a selected angle of attack relative toits direction of advance, a first anchorage means at its anteriorsurface for a towing warp, a second anchorage means at its posteriorsurface and aft of said first anchorage means, for lines which extend toa net, said first and second anchorage means being located in an uprightplane which is disposed at such angle to the general plane of the bodyas to create a moment to maintain the body at the selected angle ofattack, said anterior surface being formed as a hydrofoil having a givenconcavity fore and aft, which at its forward end makes a selected smallangle to the direction of advance, but of progressively increasing angletowards its after end, and the bodys posterior surface having aconvexity corresponding generally to the shape of the anterior surface,and auxiliary hydrofoil surface means spaced aft of the posteriorsurface, and located to intercept eddy cur-rents that may tend to breakaway from said posterior surface, and to direct the same again towardsthat surface, towards the after edge thereof.

6. A trawl door comprising a nonbuoyant body generally of hydrofoilshape fore and aft, for disposition uprightly in use, at selected andincreasing angles of attack relative to its direction of advance fromits forward towards its after ends, respectively, anchorage means at itsanterior surface for a towing warp, and upright auxiliary hydrofoilsurface means supported from and spaced aft of the bodys posteriorsurface, and towards its after end, in position to intercept eddycurrent that may tend to break away from the posterior surface, and todirect the same again towards that surface, and towards the after endthereof.

7. A trawl door for use in towing and spreading a wing of a trawl net,comprising a nonbuoyant body of generally concave-convex shape fore andaft, anchorage means at its anterior face for a towing warp, saidanchorage means being located in effect somewhat nearer the lower edgethan the upper edge of the body, to counteract the drag of the bottomupon its lower edge, and to maintain the door generally upright in use,said body having upper and lower bands at opposite sides of itslongitudinal center line which are directed longitudinally and arerelatively inclined dihedrally and rearwardly.

8. A trawl door comprising a nonbuoyant body which in use is disposedgenerally upright, having an anterior surface disposed at a selectedangle of attack to the direction of advance, and a posterior surfacecurved convexly from a leading edge which is nearly tangent to therelative current towards a trailing edge which is at a relatively steepangle to the relative current, and at least one auxilliary surfaceoutstanding behind the posterior surface 10 of the body, at an angle tothe relative current which follows the curvature of the leading edgeportion of such posterior surface, and located to intercept such currentwhere the same tends to break away from the posterior surface, and toreturn the same towards the posterior surface and the trailing edgeportion thereof.

9. A trawl door for use in spreading a wing of a trawl net, comprising anonbuoyant body which is generally upright in use, said body having agenerally upright central band and laterally spaced upper and lowerbands extending horizontally of its anterior face, and said upper andlower bands each being inclined rearwardly at an angle to the bodysgeneral upright plane, anchorage means affixed to its anterior face fora towing warp, said anchorage means being located in effect somewhatnearer the lower edge of the body than its upper edge, thereby tendingto tilt the door rearwardly at its upper edge by the pull of the towingwarp, although still remaining generally upright, in use.

10. A trawl door as in claim 9, wherein the areas of the upper and lowerbands are generally equal, to produce equal and opposite tilting effectswhen the door is exactly upright, but the below-center location of thetowing warp anchorage means tends to incline the door as a wholerearwardly at its upper edge, in use, to the extent that water flow overthe rearwardly slanted upper band produces a downward force which helpsto hold the doors lower edge on the bottom.

11. A trawl door as in claim 10, wherein the location of the towing warpanchorage means is sufiiciently below a longitudinal center line of thedoor to counter the bottom drag force upon the lower edge of thedoor,'and still to maintain the door tilted rearwardly.

12. A trawl door as in claim 9, wherein the door is of hydrofoil shapein the longitudinal direction, and further includes means affixed to itsposterior face for intercepting and redirecting eddy currents towardsthe doors said posterior face.

13. A trawl door as in claim 9, wherein the towing warp anchorage meansis located slightly ahead of the longitudinal midpoint of the door.

14. A trawl door as in claim 9, wherein said central band is generallyparallel to the doors general plane.

15. A trawl door as in claim 9, further including anchorage means on thedoor for a rearwardly extending sweep line means, and wherein theeffective locations of the respective anchorage means for the towingwarp and the sweep line means are such as to maintain the door at aselected angle of attack and to equalize the longi tudinally extendingvertical stresses on the door, to maintain its lower edge in generallyequal contact with the bottom throughout its length.

16. A trawl door comprising a nonbuoyant body ballasted to remaingenerally upright in use, said body having a bracket at its anteriorface, said bracket having a pair of bracket ears spaced equally aboveand below the longitudinal center line of said body, an anchorage for atowing warp comprising a sleeve disposed uprightly between said bracketears and releasably pivoted thereon, and an eye projecting from saidsleeve, and located nearer to one end of the sleeve than to the otherend.

17. A trawl door for trawl nets comprising a nonbuoyant hydrofoil bodyfor disposition uprightly in use with its general plane at a selectedangle of attack, said body including a central longitudinal banddisposed uprightly in use, and upper and lower bands each disposeddivergently rearwardly from said central band at obtuse vertical angles,and said body in its three bands being formed as a concave-convexhydrofoil, and one anchorage means for a forwardly directed towing warpand another for a rearwardly directed line at the net, said anchoragemeans being disposed in an upright plane one at the bodys anteriorsurface and the other at its posterior surface, and

the two being located at an angle to the general plane of the body, tocreate a moment during use, to maintain the body at the selected angleof attack.

18. A trawl door as in claim 17, the auxiliary hydrofoil surface meanscomprising a plurality of hydrofoil vanes spaced successively aft of thebodys posterior surface, and horizontally disposed brackets supportingsaid vanes from the body.

19. A trawl door as in claim 18, wherein the posterior anchorage meansare mounted on the respective brackets, and the anterior anchorage meansis located intermediate the upper and lower brackets, at the anteriorsurface of the body.

20. A trawl door as in claim 18, and a protective pad mounted at thelower surface of the lower bracket,

1 2 References Cited by the Examiner UNITED STATES PATENTS 2,942,3711/1960 Johnson et al 439 FOREIGN PATENTS 1,260,072 3/ 1961 France.

26,332 11/1 896 Great Britain. 223,500 10/ 1924 Great Britain. 523,4527/ 1940 Great Britain. 533,121 2/1941 Great Britain. 168,471 6/ 1934Switzerland.

SAMUEL KOREN, Primary Examiner.

W. H. CAMP, Assistant Examiner.

1. A TRAWL DOOR COMPRISING A NONBUOYANT BODY WHICH IS GENERALLY UPRIGHTIN USE, AND AT LEAST THE POSTERIOR SURFACE OF WHICH IS OF HYDROFOILSHAPE IN THE LONGITUDINAL DIRECTION, AND AUXILIARY SURFACES LOCATEDBEHIND THE POSTERIOR FACE OF THE DOOR IN POSITION TO INTERCEPT INCIPIENTEDDY CURRENTS AND TO DIRECT THEM TOWARDS SUCH POSTERIOR FACE AND ITSTRAILING END.